11-12高二上·黑龙江大庆·期末
解题方法
1 . 用平行于四面体
的一组对棱
、
的平面截此四面体(如图).
![](https://img.xkw.com/dksih/QBM/2011/3/24/1570061440770048/1570061446004736/STEM/94727a00-a4fa-44d6-b449-ab5f4e9d0f5a.png?resizew=151)
(1)求证:所得截面
是平行四边形;
(2)如果
.求证:四边形
的周长为定值.
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/411b38a18046fea8e9fab1f9f9b80a5f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f52a58fbaf4fea03567e88a9f0f6e37e.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/9d78abbad68bbbf12af10cd40ef4c353.png)
![](https://img.xkw.com/dksih/QBM/2011/3/24/1570061440770048/1570061446004736/STEM/94727a00-a4fa-44d6-b449-ab5f4e9d0f5a.png?resizew=151)
(1)求证:所得截面
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/389bc3f29c058067e06e0d0d2be399da.png)
(2)如果
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/1703c9549330198bccb64a1d226eae32.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/389bc3f29c058067e06e0d0d2be399da.png)
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